The present invention is related to a process for diffusing Cd dopant into an InP substrate within an evacuated and sealed tube. The volume of the diffusion chamber of the sealed tube used in the present invention can be much larger than 10 cc. and may be 150 cc. or more. The invention thus can attain the simultaneous diffusion of a large number of InP substrate wafers, rendering it possible to use the sealed tube diffusion technique in the mass production of Cd-diffused InP substrate wafers.
Optical fiber communication will become one of the primary mediums for long-distance and mass-information communication. For producing the laser diodes, light emitting diodes and photo-detectors utilized in the SiO.sub.2 -based optical fiber system, the InP/InGaAsP material system is often adopted because the SiO.sub.2 -based optical fibers have the lowest optical loss at the wavelengths of 1.3 and 1.55 micrometers, while the material system has an energy gap range from 1.0 to 1.6 micrometers which cover the two lowest loss values. The optoelectronic devices produced with the material system have thus been widely used in the illuminating sources and photo-receivers of the optical fiber communication system.
In the production of the optoelectronic devices, the vapor phase diffusion method is used to diffuse the atoms of Group II elements, such as Zn and Cd, into InP wafers to form p-n junctions or local areas of high hole carrier concentration to facilitate the formation of ohmic contacts. The method is a critical and important technique because the Zn- or Cd-diffusion can form high quality p-n junction or high hole concentration areas which in turn have a critical influence on the properties of the optoelectronic elements. Compared with Cd dopant, Zn dopant's diffusion speed is higher, its diffusion control is more difficult and the resultant p-n junction depth is less uniform. These defects of Zn-diffusion become more serious when a shallow p-n junction depth (e.g. less than a few micrometers) is desired and when the volume of the diffusion chamber of the sealed tube is increased. In contrast, Cd-diffusion has the advantages of easy diffusion control, uniform p-n junction depth and good reproducibility. Therefore, it is advantageous to use Cd as the dopant when a tube having a large-volume chamber is used. In the mass production of optoelectronic devices, it is desired that the mirror-like surface of the wafer can be maintained after diffusion, the physical and electrical properties of the diffusion layer have good controlability and reproducibility, and a large number or a large area of wafers can be diffused simultaneously for economic benefits. Therefore, the provision of a Cd-diffusion technique which is capable of mass producing diffused wafers in a scaled-up diffusion chamber and has good controlability and reproducibility on diffusion results is highly desirable for the production of InP-system optoelectronic devices.